Hypophosphatasia is a rare heritable disease caused by mutations that affect the activity of the tissue non-specific alkaline phosphatase (TNAP) isozyme. The clinical severity varies widely from complete absence of bone mineralization and stillbirth in the perinatal/infantile form to spontaneous fractures and loss of decidual teeth in adult and odontohypophosphatasia. While hypophosphatasia is usually inherited as an autosomal recessive trait, variable expressivity, incomplete penetrance and autosomal dominant inheritance have been reported. Several missense mutations in the TNAP gene have been reported in hypophosphatasia and compound heterozygosity appears frequently. No treatment is currently available for hypophosphatasia. Recently, we have produced and characterized a mouse model of infantile hypophosphatasia generated by introducing a null allele in the TNAP locus. In this grant application we propose to test the hypotheses that A) the apparent complex genetics of hypophosphatasia, results from mutations affecting different functional domains in the TNAP molecule and B) that autologous transplantation of ex vivo manipulated bone marrow osteoprogenitor stem cells may be a viable treatment modality for hypophosphatasia. Consequently, the specific aims are: 1) To study and compare the structural and functional disturbances in the TNAP molecule caused by those missense mutations that have been found associated with the perinatal/infantile, adult and odontohypophosphatasia. 2) To generate animal models of adult and odontohypophosphatasia in order to understand the pathogenesis and mode of inheritance in comparison to our perinatal/infantile hypophosphatasia model. 3) To introduce AP expression constructs into bone marrow osteoprogenitor stem cells derived from our hypophosphatasia mice and analyze the ability of these ex vivo manipulated stem cells to correct the hypophosphatasia phenotype. Our work will provide information about the molecular mechanism(s) of pathogenesis of hypophosphatasia and the molecular basis for the different genetic modes of, transmission. Hypophosphatasia will be used as a paradigm for genetic bone diseases as we test the feasibility of using genetically manipulated mesenchymal stem cells in the treatment of this disease.